As described in Japanese patent application Kokai publication No. Hei 11-51582 (Patent Document 1), use of a Rankine cycle has heretofore been proposed to reduce fuel consumption by recovering waste heat from an internal combustion engine. In order to efficiently operate the Rankine cycle, it is necessary to adjust an evaporation temperature of a refrigerant by changing a pressure thereof. A fluorocarbon-based refrigerant used in a waste heat recovery Rankine cycle gasifies at a low temperature, and therefore has a feature that the refrigerant set at an appropriate pressure allows a Rankine cycle to operate with a heat source at a low temperature equal to or less than 100° C. On the other hand, this technique has a drawback that it is not possible to make the temperature of the refrigerant higher than its thermal decomposition temperature.
During recovery of waste heat of exhaust gas from an internal combustion engine of an automobile or the like, the temperature of the exhaust gas varies significantly in a range from 100° C. to 800° C. depending on a driving condition. Further, when the temperature of the exhaust gas is high, an exhaust gas flow rate also increases in proportion thereto. As a consequence, an amount of waste heat also becomes very large. In the meantime, regarding waste heat from cooling water in the internal combustion engine, the temperature of the cooling water varies less significantly in a range from 80° C. to 100° C., whereas an amount of the waste heat varies significantly.
In the case of recovering the waste heat whose amount of heat varies significantly as described above, a flow rate of the refrigerant should be changed appropriately in order to maintain the temperature of the refrigerant in the Rankine cycle at the evaporation temperature, which is equal to or less than the thermal decomposition temperature and is under a pressure that enables efficient operation.
However, in the conventional structure of the waste heat recovery Rankine cycle provided with one expander and one pump only, it has been difficult to change the flow rate of the refrigerant in such a way as to maintain a constant temperature while keeping the pressure at a prescribed value.